The present invention relates in general to paper making machines and, more particularly, to a two-wire forming section of a paper making machine including a forming shoe having a curved guiding surface for guiding the joint run of the forming wires and web interposed therebetween over a curved path of travel whereby dewatering of the web takes place in the region of the forming shoe at least in the direction away from the forming shoe, the dewatering being assisted by centrifugal forces arising from the curvature of the forming shoe.
It is well known in the art of paper making machines that the dewatering of a web being manufactured on a standard Fourdrinier paper machine is entirely in the downward direction, whereby fines and fillers depart from the wire side of the web due to the washing action of foils or table rolls. For this reason, the two sides of a web manufactured on a standard Fourdrinier machine exhibit anisotropic properties, the topside of the web having a greater smoothness and containing greater amounts of fines and fillers, while a lesser amount of fines and fillers are present on the wire side of the web. Moreover, the wire side of the web is often subjected to marking as is well known. For these reasons, paper manufactured by means of two-wire forming sections is considered preferable, particularly as regards its printability characteristics.
Two-wire forming sections are known in the art which do not include stationary dewatering elements. However, such conventional two-wire formers give rise to poor web formation since, among other reasons, there is no provision for pulsations of the dewatering pressure which have been found to contribute to better web formation. It is a further drawback of these conventional machines that it is not possible to control the relative proportions of dewatering through the upper and lower wires, respectively. It has been stated that such control would be desirable.
Two-wire forming sections which incorporate so-called forming shoes as stationary dewatering elements are also known. Such forming shoes are generally fixed in position. The conventional forming shoes usually present a guiding surface facing the wire having the same structure over its entire length in the direction of travel of the joint run. A forming shoe of this type has been found to operate in an optimal manner only at one particular speed of the paper machine and for one particular brand of paper being manufactured.
Regarding the state of the art to which the present invention pertains, reference is made to U.S. Pat. No. 4,154,645, issued May 15, 1979 wherein a method and web-forming unit for manufacturing multi-ply cardboard are disclosed and wherein a forming shoe is provided on the two-wire run. The guiding surface of the forming shoe has an open and closed construction wherein the length of the open and closed guiding surface is arranged to be adjustable by means of filler lists so that a slitted surface of the forming shoe can be converted into a totally closed guiding surface. This forming shoe is specifically intended for manufacturing multi-ply cardboard and it is noted that the position of the shoe cannot be adjusted.
Recently, planar wire machines have been modernized by rebuilding or retrofitting them by arranging one or more upper wire units over the planar wire unit in order to achieve a dewatering of the web in an upward direction so as to both increase the dewatering capacity as well as to improve both the formation and retention characteristics. An increase in the dewatering capacity enables the speed of the paper machine to be increased. Moreover, such constructions also enable the concentration of the pulp stock supplied from the headbox to be reduced which is advantageous in itself. In certain instances, old and slow newsprint machines have been modernized by the provision of such upper wire units to enable them to operate as cardboard machines without increasing the machine speed.